1. Field of the Invention
The present invention relates to a surface scan measuring device, and a method of forming a compensation table for a scanning probe. More particularly, the present invention relates to a surface scan measuring device that scans the surface of a workpiece to measure surface roughness, waviness, profile, etc. of the workpiece.
2. Description of Related Art
There are diffused surface scan measuring devices that scan the surface of a workpiece to measure surface texture and three-dimensional profile of the workpiece, and there are known a machine for measuring surface roughness, a machine for measuring profile, a roundness measuring machine, and a coordinate measuring machine.
FIG. 17 shows a schematic view of a conventional measuring system 100 that is a surface scan measuring device using a scanning probe.
The measuring system 100 includes a coordinate measuring machine 1 for moving a scanning probe 2, an operation unit 3 having a joystick 31 for use in manual operation, a motion controller 4 for controlling the operation of the coordinate measuring machine 1, a host computer 5 for operating the coordinate measuring machine 1 through the motion controller 4 and processing measured data of a workpiece W obtained by the coordinate measuring machine 1 to obtain dimensions and profile of the workpiece W.
The scanning probe 2 has a stylus 21 whose leading end is provided with a contact portion (measuring piece) 22, and a holding portion 23 that slidably holds the basal end of the stylus 21 along Xp direction, Yp direction, Zp direction within a predetermined range, as shown in FIG. 18.
The holding portion 23 has a slide mechanism, not shown, which is provided with an xp slider, a yp slider, and a zp slider that can slide along directions perpendicular to each other, and a probe sensor 24 (shown in FIG. 1) that detects displacements of the slide mechanism along respective axis directions and outputs thus detected displacements. The stylus 21 is slidably held by the slide mechanism in a predetermined range on the basis of the holding portion 23.
The configuration of the scanning probe is disclosed in Japanese Patent Laid-Open Publication No. JP05-256640.
Thus configured scanning probe 2 is made to scan the surface of a workpiece with its contact portion 22 abutting on the surface of the workpiece by a reference displacement amount Δr.
At this time, a motion trajectory of the scanning probe 2 is obtained using drive amount of the coordinate measuring machine 1. The motion trajectory of the scanning probe 2 corresponds to a motion trajectory of the contact portion 22, and the contact point between the workpiece surface and the contact portion 22 is located at a position offset from the center of the contact portion 22 by radius “r” of the contact portion 22.
The scanning probe 2 is made to scan the surface of the workpiece with its contact portion 22 pressed to the workpiece surface by the reference displacement amount Δr.
FIG. 19A and FIG. 19B show views for explaining the state in which the contact portion 22 is pressed to a workpiece W by the reference displacement amount Δr. FIG. 19A shows the state in which the contact portion 22 is in contact with workpiece W, and the reference displacement amount Δr is zero. In this state, the distance between the center P1 of the contact portion 22 and the abutting point of the workpiece W is equal to the radius “r” of the contact portion 22. In this case, since the reference displacement amount Δr is zero, it is not determined whether or not the contact portion 22 of the scanning probe 2 is in contact with workpiece W.
FIG. 19B shows the state in which the contact portion 22 of the scanning probe 2 is pressed to the workpiece W so that the reference displacement amount Δr is set to be a predetermined value. In this case, since the contact portion 22 is pressed to the workpiece W with a pressure determined by a measuring force, the stylus 21 is bent.
Consequently, the distance between the center P2 of the contact portion 22 and the abutting point of the workpiece W is equal to the radius “r” of the contact portion 22, which is similar to the case shown in FIG. 19A. On the other hand, the distance between the detecting position of the probe sensor 24 (a position shifted by the reference displacement amount Δr as compared with the case shown in FIG. 19A, or the apparent center P3 of the contact portion) and the abutting point of the workpiece W is set to be an offset value Q, which value is different from the radius “r” of the contact portion 22. Accordingly, the offset value Q is different from the radius “r” of the contact portion 22 due to above-described flexure of the stylus 21 and, other than this, the influence of the sphericity of the contact portion 22.
When a motion trajectory drawn by the apparent center P3 of the contact portion is corrected by the offset value Q toward the workpiece surface, the surface profile of the workpiece can be obtained.
The reference displacement amount Δr is not required to be the same value all the time, and may be within a proper reference position range in the measurable range of the probe sensor 24.
There are raised errors in detecting displacements of the stylus 21 by the probe sensor. Then, errors are brought about in the displacement amount, which leads to measurement errors. So as to correct detection errors by the probe sensor, compensation coefficients are set up depending on the detection property of the respective axes (Xp direction, Yp direction, Zp direction). For example, a compensation coefficient of Kxp′ is set up for the Xp direction, a compensation coefficient of Kyp′ is set up for the Yp direction, and a compensation coefficient of Kzp′ is set up for the Zp direction. When detected values of the respective axes by the probe sensor are Xp, Yp and Zp the detected values Xp, Yp and Zp by the probe sensor are corrected as follows.Xd=Xp·Kxp′Yd=Yp·Kyp′Zd=Zp·Kzp′
The Xd, Yd and Zd are values obtained by correcting output values of the probe sensor using the compensation coefficients Kxp′, Kyp′ and Kzp′.
However, in some cases, detection errors cannot be sufficiently corrected by only setting up the compensation coefficients Kxp′, Kyp′ and Kzp′ for the respective axes, and correcting output values of the probe sensor using the compensation coefficients. Consequently, when measuring a perfect circle, the measurement result may come to an ellipse due to measurement errors. That is, detection errors cannot be sufficiently corrected by only multiplying the detected values by the compensation coefficients set up for the respective axes.
Furthermore, in some cases, the surface profile of the workpiece cannot be obtained accurately by correcting the motion trajectory drawn by the apparent center P3 of the contact portion by the offset value Q alone. The cause of the errors is attributed to the flexure of the stylus 21 brought about when the contact portion 22 of the scanning probe 2 is pressed to the workpiece W. That is, it is thought that the offset value Q varies when the stylus 21 is bent.
Because of the problem, profile measurement by the surface scan measuring device includes errors and the errors cannot be sufficiently corrected, which leads to difficulty in improving accuracy of profile measurement.